Conventional fixed spectrum allocation schemes lead to significant spectrum under-utilization. For instance, experiments have indicated that as much as 62% of the spectrum below the 3 GHz band is idle even in the most crowed areas (e.g., near downtown Washington D.C. where both government and commercial spectrum usage is intensive). Such under-utilization and increasing demand for the radio spectrum suggest that a more effective spectrum allocation and utilization policy is necessary.
Currently, secondary users may use the licensed spectrum only when such use does not interfere with a primary user's communication. Such requirements imply that the use of the spectrum varies with time (i.e., dynamically changes), and is dependent on the load imposed by the primary users. Several conventional systems use multiple channels in wireless networks. However, such systems fail in dynamic spectrum access networks. For example, conventional systems assume that the set of available channels is static (i.e., the channels available for use are fixed at the time of network initiation). In multiple access wireless networks, however, such as Worldwide Interoperability for Microwave Access (WiMAX) with orthogonal frequency division multiple access (OFDMA) signaling, the set of available channels dynamically changes. Furthermore, conventional systems often assume that the available channels are “homogeneous” (i.e., different channels have similar range and support similar data rates). These assumptions do not hold true in situations such as, for example, where different channels are located on widely separated slices of frequency spectrum with different modulation schemes and different propagation characteristics.
Therefore, there is a need for synchronous spectrum sharing systems based on orthogonal frequency-division multiplexing (OFDM) or OFDMA signaling.